Model checking is a successful technique for verifying automatically temporal properties of concurrent finite-state programs represented as Labelled Transition Systems (LTSs). Among the various formalisms used for specifying properties, an outstanding candidate is the modal mu-calculus, a very powerful fixed point-based temporal logic. However, in spite of its theoretical expressiveness, standard modal mu-calculus is a too low-level formalism for end-users, making the specification of complex properties tedious and error-prone. In this talk, we propose MCL (Model Checking Language), an extension of the modal mu-calculus with high-level, data-based constructs inspired from programming languages, which substantially increase its expressive power as well as the conciseness and readability of properties. We also present an on-the-fly model checking method for verifying MCL formulas on LTSs, based upon translating the verification problem into the resolution of a boolean equation system. The MCL language and the associated verification method are supported by the EVALUATOR 4.0 model checker, developed within the CADP verification toolbox using the generic OPEN/CAESAR environment for on-the-fly exploration of LTSs.